This invention relates to a riser impact protection.
Substantial effort is spent in seeking hydrocarbons under water. The drilling rig is provided with a drilling riser which typically comprises a central large diameter pipe (which may be of the order of 0.5 to 0.6 m). The riser acts as a conduit for a drillstring between the drilling rig and the seabed BOP/wellhead assembly. Additionally drilling fluid and cuttings are transported back up the riser to the surface, having been pumped down the drillstring, for processing and reclamation of the drilling fluid. Around the central pipe there is typically provided a plurality for example 2, 3 or 4 smaller risers. These risers handle other process fluids to assist with control operations such as running and shutdown.
With the depletion of reserves in shallow and relatively calm water, there has become a move towards exploration of deep and/or rough water. In deep waters of the order of 600 m or more, a simple riser can be difficult to use. First, the suspended weight of the riser may be such as to affect the stability of the drill ship. Secondly, the weight of depending riser may be so great that towards the top of the riser assembly the material may be damaged as the tensile properties are exceeded.
Conventionally to overcome this problem much of the weight of the riser assembly is offset by buoyancy attached to it. Typically, around 65 to 90% of the submerged weight is offset by the use of buoyancy. The buoyancy may take the form of strapped-on modules of syntactic foam. Examples of such modules are described in our GB 2 286 649.
While buoyancy offsetting around 65 to 90% of the submerged weight of the suspended string is typically used, the buoyancy is not applied uniformly over the length of the drillstring. Typically, a substantial part of the bottom of the drill string is not xe2x80x9cdressedxe2x80x9d. Leaving the bottom portion xe2x80x9cundressedxe2x80x9d provides a stabilising weight for the drillstring prior to connection to the BOP/wellhead assembly. Additionally because the surface portion is typically subject to the highest current velocity it may be left undressed to reduce drag and riser deflection.
The drillship is provided with a xe2x80x9crotary tablexe2x80x9d through which the riser joints pass. The rotary table is a steel structure with a hole through which the riser module assembly passes. A working clearance is provided between the module assembly and the hole. Nevertheless, during operation subsurface currents impinging on the suspended riser deflect it from vertical. Surface waves also cause the drillship to move relative to the riser. These relative movements can cause the assembly to abut the hole in the rotary table. Where the riser is dressed the energy of the impact is taken by the buoyancy material which may suffer some damage. However in most cases where the riser is dressed the smaller pipes, and the main riser are protected from damage. Where, however, the riser is undressed this protection is not present and substantial impact damage can occur.
U.S. Pat. No. 4,004,531 seeks to reduce the relative movement of the riser string and the drillship by providing the drillship with a downwardly depending caisson which surrounds the riser but is not attached to it. The caisson is provided with a plurality of holes. The intention is that the force of waves and currents impinging on the caisson is reduced and therefore reduces the relative movement of drillship and riser.
The invention seeks to provide a way of reducing the likelihood of damage without providing unwanted and expensive additional buoyancy.
In accordance with the invention the problem is solved by providing a hollow shell which floods on immersion with water. The shell which should be impact resistant is air-filled when out of the water and therefore relatively easy to handle. Following immersion the shell floods giving a structure of broadly neutral buoyancy.
According to an aspect of the invention there is provided a shell for providing impact protection to a marine drilling riser, the shell comprising a body having at least one hollow chamber, the body being provided with a plurality of holes allowing flooding of the chamber. The shell can comprise a plurality of shell elements joined together. The shell can be rotationally moulded, blow moulded, injection moulded or fabricated in other ways. The shell can comprise thermoplastics material such as polyethylene or it can comprise a thermosetting resin composite or a thermoplastic composite. A plurality of tubes may extend through the chamber. At least one tube can be solid and at least one tube can be filled for example with a material having a high compressive modulus such as hard polyurethane elastomer. The shell elements can be joined together by one or more bolts or straps which may comprise stainless steel, titanium or Kevlar(copyright) (polyparaphenyleneterephthalate). The shell can have a length in the range 0.5 to 5 m preferably 1 to 4 m. The shell together with any straps and thrust collars can be broadly of neutral bulk density when submerged in seawater for example having a bulk density in seawater within the range of 0.95 to 1.05 times the bulk density of seawater. In some embodiments of the invention one of the inner face and the outer face is provided with furrows directed into the chamber and the other of the outer face and the inner face is provided with co-operating projections directed into the chamber.